In a move that elicited little surprise but much vexation, the Service Employees International Union (SEIU) — one of the more progressive unions in organized labor, principally representing public sector, health care, and building workers — announced last week it was endorsing Hillary Clinton over Bernie Sanders in the Democratic presidential primary.

SEIU has already spent some $30 million in support of Fight for 15, the organizing campaign to raise the minimum wage to $15 an hour and unionize low-wage workers. But whereas Sanders has unequivocally declared his support for a national $15 minimum wage, Clinton has said she only favors a $12 wage floor.

Why, then, did the two-million-member union go for Clinton? And what does it say about organized labor in the present and future?

The best way to understand SEIU’s decision is to work backwards from what it considers an absolute imperative: electing a Democratic president in 2016. For the labor movement, it is now a question of survival. Union membership represents barely 11 percent of the American workforce, and that number drops nearly every year. A Republican chief executive means a Republican majority on the National Labor Relations Board, and perhaps decades more of a right-wing Supreme Court, which has already shown itself willing to disrupt generations-old norms in workplace representation.

If, as seems likely, the GOP hold its congressional majorities, then a Republican president could mean that the eighty-year-old National Labor Relations Act, which grants to workers the right to bargain collectively, will itself face an existential threat. In the general election, SEIU, and the rest of the US labor movement, will therefore back the Democratic nominee to the hilt — whoever it turns out to be.

But we’re still in primary season, and SEIU, along with most of the American political class, presumably reasons that Sanders can’t win a general election. Never mind that polls show nothing of the sort; never mind that Clinton’s negatives are disturbingly high. People who do politics for a living know in their bones that a candidate proclaiming himself a democratic socialist can’t become president.

If Sanders can’t win, and Clinton has to, then the union’s priority becomes expediting Clinton’s victory in the primary so she can start running a general election campaign months before the Republicans settle on their nominee.

Furthermore, Clinton already has the backing of unions that together represent about two-thirds of American union members (including the two national teachers unions and the largest union of public-sector workers). SEIU might have calculated that Sanders would need the bulk of organized labor behind him to defeat the Clinton juggernaut.

Then there’s the business of access. The massive resources SEIU poured into Barack Obama’s election bid in 2008 won President Andy Stern and Secretary-Treasurer Anna Burger priority admittance to Obama’s White House, where they played an outsized role in representing the labor movement’s position on health care reform and the Employee Free Choice Act.

If Clinton is going to win — because she has to win — then delaying a primary endorsement has no upside. The union would simply jeopardize its spot on Clinton’s crowded list of favors to return.

But the access argument is also unpersuasive. In 2007 the union was divided internally over whether to back John Edwards or Obama. In the end the national union allowed its state affiliates to go their separate ways, only uniting behind Obama after Edwards had dropped out after the first round of primaries. Opting not to come out early for Obama didn’t prevent the union from mobilizing members and resources for the general election. Similarly, SEIU will be indispensable to the Democratic nominee’s chances in November, so it is hard to argue that Clinton could shut the union out.

Finally, there is the question of where the rank-and-file stands. An SEIU source told Politico that polls of its members “consistently showed Clinton obtaining more than 72 percent of union members’ support.” Some of this reflects the fact that many Democrats, especially black and Hispanic voters, still aren’t familiar with Sanders. Clinton, meanwhile, has always been broadly popular with Democratic voters.

It’s not hard to see how this could have played out differently. For example, what if SEIU had sponsored a candidates forum specifically on workers issues, with questions coming directly from members, and then asked members who they thought the union should support?

At the same time, given that the default position was always going to be a Clinton endorsement, there would have had to be a bigger rank-and-file upsurge for Sanders. In its mega-locals, SEIU is an inhospitable place for bottom-up movements looking to challenge leadership on political strategy; nevertheless had there been a visible demand for Sanders from large numbers of members it could have at least slowed SEIU’s endorsement timeline and given Sanders more of an opportunity to make his case to the membership.

Absent such a push, the SEIU leadership was happy to lend the union’s imprimatur and declare confidently that Clinton will “fight, deliver, and win for working families” — irrespective of her record (e.g. championing welfare reform), past affiliations (a stint on the board of Walmart), spectacular personal wealth ($235,000 per speaking engagement, forty times a year), or her chief donors (Wall Street and the medical-industrial complex).

Comments from SEIU’s largest local suggest the union is perfectly happy to see Sanders pressing Clinton to take more left-leaning positions. But the labor movement still sees the election solely through the prism of its outcome — not in terms of what Sanders’s candidacy represents, or makes possible.

That narrow electoralism could end up harming Fight for 15 — not just the union’s most important campaign, but arguably the most important labor battle happening today. SEIU’s decision to provide the financial largesse for Fight for 15 comes from the indisputably correct observation that unless the labor movement can bring millions of low-wage workers into its fold, organized labor is scheduled for expiry.

Yet before the endorsement announcement, SEIU President Mary Kay Henry toldAl Jazeera that though the union is expecting “candidates up and down the ticket who are willing to get in the streets and champion this demand,” support for a $15 minimum wage is not a “litmus test” but an “aspirational demand.”

Over the last three years, SEIU has spent tens of millions of dollars and galvanized the labor movement around an inspiring fight. It has justified this enormous expenditure to its members by correctly arguing that they won’t be able to protect and improve their own standards unless something is done to boost the wages of the worst paid workers.

But if the union actually believed it could win on this issue — if it believed it could lead — then a litmus test is exactly what it would be. Clinton would just have to get in line. Members and non-members have shown that they are willing to fight for $15 and a union. What does it say to them if they now are asked to knock on doors calling for $12 and a Clinton?

The contrast with the British labor movement is striking. In the recent contest to choose the next leader of the UK Labour Party, the country’s largest unions opted to endorse the socialist Jeremy Corbyn — when he was still in fourth place, and despite shrill Blairite cries that he was unelectable. While Corbyn ended up trouncing his opponents, much (though not all) of US labor has displayed no such willingness to back its own left insurgent candidate.

At the very least, a Sanders endorsement would have signaled that mainstream Democrats actually need to work to win labor support. Instead, the decision betrayed a defensiveness and myopia — both of which have contributed to labor’s decline.

A Democrat in the White House may be necessary to keep the most reactionary anti-labor forces out. But staving off that fate will be the best the labor movement can hope for unless ordinary people mobilize in great numbers — for a reinvigorated democracy, and for a more just economy.

The widespread support for Sanders and his “political revolution” — not just his $15 minimum-wage pledge — clearly indicates that many Americans are moving in that direction. In remaining aloof from it, SEIU and the bulk of the US labor movement are passing up the chance to harness that energy into something even bigger and more transformative.